Timekeeping on the Moon is an issue of synchronized human activity on theMoon and contact with such. The two main differences to timekeeping on Earth are the length of a day on the Moon, being thelunar day orlunar month, observable from Earth as thelunar phases, and the rate at which time progresses, with 24 hours on the Moon being 58.7microseconds (0.0000587 seconds) faster,[1] resulting fromgravitational time dilation due to the different masses of the Moon and Earth.
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The technology used for thetimekeeping devices deployed to the Moon have varied over the decades. SeveralOmega Speedmasters have been on the Moon, synched toCentral Standard Time (CST).[2]
TheApollo Guidance Computer (AGC) kept a triple-precision count of time in areal time clock cuing from aquartz oscillator; a standby option (although never used) would allow it to update this count every 1.28 second (~0.78 hertz) — more often when not standing by. In addition to maintaining theclock cycle,computer timekeeping allowed the AGC to display the capsule's vertical and horizontal movements relative to the Moon's surface, in units offeet per second.[citation needed]
Coordinated Lunar Time (LTC) is a proposed primary lunar time standard for theMoon.[3] In early April 2024, theWhite House askedNASA to work alongside US and international agencies for the purpose of establishing a unified standard time for the Moon and other celestial bodies by 2026.[4] The White House's request, led by theOffice of Science and Technology Policy (OSTP), called for a "Coordinated Lunar Time", which was first proposed by theEuropean Space Agency in early 2023.[3][5]
There is no lunartime standard. As a result, activities on the Moon are coordinated using the time zone of where a mission's headquarters is based.[6] For example, theApollo missions utilized theCentral Time Zone as the missions were controlled from Houston, Texas.[7] Likewise,Chinese activities on the Moon run onChina Standard Time.[citation needed] As more countries are active on the Moon and interact with each other, a different, unified system will be needed.[7]
With renewed international interest in human travel to the moon, reminiscent of thespace race, especially in the United States and China,[8][9] a need exists for a universal time-keeping benchmark so that lunarspacecraft andsatellites are able to fulfill their respective missions with precision and accuracy.[10][11] Due to differences ingravitational force and other factors, time passes fractionally faster on the Moon when observed from Earth.[12][13]
Under theArtemis program, and supported by theCommercial Lunar Payload Services missions, astronauts and a proposed scientificmoonbase are envisioned to take place on and around the lunar surface from the 2020s onwards.[14] The proposed standard would therefore solve a timekeeping issue.[15] According to OSTP ChiefArati Prabhakar, time would "appear to lose on average 58.7 microseconds per Earth-day and come with other periodic variations that would further drift Moon time from Earth time".[16]
The development of the standard is set to be a collaborative effort, initially amongst members of theArtemis Accords, but will be meant to apply globally. The initial proposal of the standard calls for four key features:[17]
LunaNet, an upcoming lunar communications and navigation service under development with theEuropean Space Agency, calls for a Lunar Time System Standard which the LTC is meant to address.[18]
In August 2024, the USNational Institute of Standards and Technology furthered development of the proposal by releasing a draft for the standard focused on defining the framework and mathematical model. The draft takes into account the gravitational differences on the Moon and was published toThe Astronomical Journal.[19]
In December 2025, researchers at thePurple Mountain Observatory inNanjing, China, released a program that can calculate LTC. Their program is accurate to about 0.15 ns up to the year 2050.[20][21]